In a number of mammalian species the pineal gland plays an important role in seasonal reproduction. When intact Syrian golden hamsters are transferred from a long to a short photoperiod, marked decreases in serum and pituitary levels of prolactin precede both gonadal and accessory organ atrophy and detectable changes in gonadotropins and testosterone levels. Pinealectomy has been shown to prevent these phenomena. Ostensibly, diminishing photoperiods lead to the production of a pineal substance which produces the antigonadotropic effect. Although the pineal indoleamine melatonin is a strong candidate for this pineal hormone, the neuroendocrine events underlying reproductive changes in response to diminishing photoperiods have not been resolved. It is now well recognized that pituitary gonadotropin and prolactin secretion are regulated by hypothalamic norepinephrine and dopamine turnover rates, and perhaps other neurotransmitters. Changes in pituitary secretion induced by changing photoperiods and pineal activity most likely involve alterations in these controlling mechanisms. The studies proposed seek to determine whether changes in hypothalamic catecholamine turnover rates follow transfer of male hamsters from a long to a short photoperiod. Additionally, measurements of hypothalamic LHRH content, pituitary and blood levels of gonadotropins and prolactin will be made in an attempt to construct a more comprehensive overview of neuroendocrine changes induced by exposure to a short photoperiod. After these baseline data are obtained, and in order to determine a more precise role for the pineal gland, hypothalamic catecholamine turnover rates will be determined in pinealectomized hamsters after transfer from long to short photoperiod. Melatonin will be administered to hamsters maintained in longer photoperiod in order to determine whether the gonadal atrophy and hormone changes resulting from this treatment are accompanied by changes in hypothalamic catecholamines which parallel those observed in short photoperiod-treated hamsters. These experiments will provide new and important information about pineal regulation of hypothalamic catecholamines, pituitary secretion and reproduction.